The invention relates to an optical scanning device in which a light beam is focused onto a rotating recording medium. The beam is guided along the data tracks of the recording medium by a tracking regulation circuit which includes a coarse drive for adjusting the optical scanning device and a fine drive for guiding the objective lens.
In a compact disk player the recording medium, the compact disk, is scanned by a light beam. The design and function of an optical scanning device, a so-called optical pick-up, are described in Electronic Components & Applications, Vol. 6, No. 4, 1984, on pages 209 through 215. The light beam from a laser diode is focused onto the disk by lenses and reflected onto a photodetector from the disc. The data stored on the disk and the values for the focusing circuit and for the tracking regulation circuit are obtained from the output signal of the photodetector In the publication cited above the deviation of the feedback value for the focusing regulation circuit is designated "focusing error" while the term "radial tracking error" is chosen for the deviation of the feedback value from the desired value of the tracking regulation circuit.
A coil serves as the actuator for the focusing regulation circuit. The magnetic field of the coil moves an object lens along the optical axis. The focusing regulation circuit moves the objective lens to insure that the light beam emitted from a laser diode is always focused onto the disk. Using the tracking regulation circuit, also often referred to as radial drive, the optical scanning device can be shifted in the radial direction with respect to the disk. Thus, the light beam can be guided along the helical data tracks of the compact disk.
With some apparatus the radial drive includes a coarse drive and a fine drive. The coarse drive typically is a threaded spindle which drives the entire optical scanning device including the laser diode, the lenses, the prismatic beam splitter, and the photodetector. The fine drive is used to shift the light in the radial direction. The light beam can therefore be displaced a small distance, e.g. about 1 ram, along the radius of the compact disk.
The attainment of accurate reproduction of the recorded data irrespective of whether the recording is a video disc, compact disc or a magneto-optical disk, requires precise focusing of the light beam on the disk and precise tracking along the data tracks of the disk. In FIG. 1 the photodetector PD of the optical scanning device of a compact disk player is shown with three laser beams L1, L2, L3 focused onto the disk. The laser beams L2 and L3 respectively are the +1 and -1 level diffracted beams. A scanning device of this type is called, in the above named citation, a three-beam-pick-up because it operates with three light beams.
In the photodetector four square-shaped photodiodes A, B, C and D are assembled to form a square. Two rectangular photodiodes E and F lie on opposite sides of the detector. The middle laser beam L1, which is focused onto the four photodiodes A, B, C and D, generates the data signal HF=AS+BS+CS+DS and the focusing error signal FE=(AS+CS)-(BS+DS). The two outermost light beams L2 and L3, the front one of which L2 falls upon the photodiode E while the back one L3 hits the photodiode. F, generate the tracking error signal TE=ES-FS. The designations AS, BS, CS, DS, ES, and FS denote the photovoltages of the diodes A, B, C, D, E and F respectively.
In FIG. 1 the middle light beam L1 exactly follows the center of a track S and the tracking error signal TE has the value zero. EQU TE=ES-FS=0.
FIG. 2 illustrates the case where the laser beams L1, L2 and L3 are shifted towards the right from the track S. In this condition FS&gt;ES and the tracking error signal assumes a negative value: EQU TE=ES-FS&lt;0.
The actuator of the tracking regulation circuit moves the optical scanning device to the left until the tracking error signal TE becomes zero.
In the opposite case shown in FIG. 3 the laser beams are displaced from the track towards the left side and the tracking error signal is positive: TE=ES-FS&gt;0. The tracking regulation circuit moves the optical scanning device to the right until the tracking error signal TE becomes zero.
The generation of the tracking error signal utilizes the diffracting property of the pits or the preprinted track. When the light beam is reflected from the center of the track the light intensity of the circular light spot on the photodiode E or F decreases; depending on the direction from which the light beam exits the track, radially inwardly or outwardly, the light spot on the photodiode E or F becomes brighter while it becomes darker on the other photodiode. This difference in brightness is utilized by a difference amplifier D1 to generate the tracking error signal TE.
In order for the light beam to follow the data track to be scanned, which because the disc is eccentric moves radially in a staggering manner, the fine drive radially moves the objective lens so that it follows the eccentric motion of the disk. However, in order to be able to guide the light beam along the data tracks it is necessary to switch on the coarse drive continuously or in time intervals so that it can re-adjust the optical scanning device because the regulating range of the fine drive covers only about 100 data tracks. The re-adjustment of the scanning device by the coarse drive should occur in such a manner that the fine drive can operate equally in both directions That is, while scanning a data track the fine drive should be maintained in the center position so that is able to radially adjust approximately the same number of data tracks inwardly or outwardly.
With a known optical scanning device which utilizes the three beam principle the tracking error signal TE (the deviation of the feedback value from the desired value) is fed to the input of a PID controller the output of which is coupled to the actuator of the fine drive and to an integrator. The output of the integrator is coupled to the actuator of the coarse drive, an electric motor, which drives the spindle. The regulating voltage at the output of the integrator causes the coarse drive to be re-adjusted in such a way that the mean value of the regulating voltage at the output of the ID controller becomes zero. Thus, the objective lens is translated about its mechanical center position which it assumes at a regulating voltage of 0.V. The mechanical center position of the objective lens is determined by springs or other flexible elements which have to be adjusted in such a manner that the optical axis of the objective lens coincides with the optical axis of the optical scanning device. Aging of the springs, and other external forces, such as those of a vehicle, can cause the optical axis of the objective lens to shift and no longer coincide with the optical axis of the optical scanning device. The mechanical center position no longer coincides with the optical center position because it has shifted a little away from the optical center position. Accordingly, when the regulating voltage is the desired zero volts the object lens is no longer situated in the optical center position and the regulating range of the fine drive is no longer symmetrical.